The users of such beams generally desire said production to be cheap, and the current density and the total intensity of the beams produced to be large.
This is particularly true when the electron beam produced is a pulsed beam for striking a target, e.g. of tantalum, in order to generate a beam of X-rays with the X-ray beam being intended to pre-ionize a discharge chamber in a pulse gas laser. In such a laser, the gaseous optical amplifying medium is pumped by an electrical discharge and the discharge is often made uniform by pre-ionizing said medium with a pulsed beam of X-rays. The X-rays must be of sufficiently high energy to be able to penetrate without substantial attenuation into the discharge chamber through a strong sealed window. They must be sufficiently intense to ensure effective pre-ionization, and to ensure it in a short period of time, e.g. 20 nanoseconds (ns). To do this, the electron beam must have high current density, e.g. 400 amps per square centimeter (A/cm.sup.2).
The most widely used generators of high energy electrons in such an application are cold cathode pulse guns. They generally operate in a so-called "secondary" high vacuum of 10.sup.-3 or 10.sup.-4 Pascals (Pa) for example, so as to ensure that the electrons emitted by the cathode and accelerated by an electric field towards the anode are not slowed down too much by collisions with the residual gas.
The need to maintain such a vacuum increases the manufacturing and maintenance cost of such a generator and also makes it more bulky.
That is why electron generators have been proposed that operate at an ambient pressure which is low but easy to obtain and to maintain. Work performed by the Collins team at the University of the Colorado on D.C. guns using alumina or carbon cathodes has made it possible to obtain 5 keV electrons using a current of 1 A. Their cathodes are flat or concave and the cathode-anode distance is not critical beyond the cathode black space. Yield reached 50%. The ambient gas was helium at a pressure lying in the range about 10 Pa to about 100 Pa.
Such a generator is described, in particular, in an article by J. J. Rocca, J. D. Meyer, M. R. Farrel, and G. J. Collins, in the Journal of Applied Physics, 56, 790 (1984).
This generator is referred to below as a Collins generator.
Other electron guns operating in low pressure helium have been described in various articles, and in particular in the following:
C. H. H. Carmichael, R. K. Garnsworthy, L. E. S. Mathias, Re. Sci. Instrum. 44, 701 (1973).
P. A . Boklan and G. V. Kolbychev, Sov. Phys. Tech. Phys. 26, 1057 (1981).
G. V. Kolbychev and I. V. Plashnik, Sov. Tech. Phys. Lett. 11, 458 (1985).
These generators appear to be less advantageous than the Collins generator.
In the Collins generator, the slowing down of electrons due to collisions with the ambient gas is made unimportant by using an acceleration voltage which is sufficiently high (e.g. 8 kilovolts (kV) or more) in association with a pressure which is low enough for a considerable fraction of the electrons emitted by the cathode to reach sufficient energy prior to encountering a molecule of the ambient gas to ensure that its collision cross-section is greatly reduced. The term "collision cross-section" represents the probability of collision with such molecules.
Such electrons are known as "runaway electrons". Because of their low probability of being slowed down by collisions, they acquire an amount of energy as they travel from the cathode to the anode which is more than half the energy that they would have acquired in a vacuum under the same accelerating field.
Unfortunately, the high voltage and low pressure used encourage discharge between the cathode supports and the anode, thereby making it impossible to obtain a stable diffuse discharge having high current density in the cathode-anode gas. It is therefore necessary to provide electrically insulating coatings on the anode and cathode supports, thereby complicating generator construction.
The object of the present invention is to produce high energy electrons in a manner which is simpler and cheaper, while using very high current densities, at least instantaneously, together with production efficiency close to that obtained in a secondary vacuum.